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Yes but what if someone with a score of 30 has something really useful to say on the post, that might help to author in someway, but they can't comment if the author sets their slider to 40. Surely having the ability to downvote spammers is enough?

There's a lot of grey area, (middle ground), between spam and valuable comments.
I guess the flipside to your example is, 'What if someone with something really valuable to say is never read because they're buried under 100 "excellent post, please follow me" copy/paste jobs?"

SO how about a flag for the comments then? Might be a better solution?

That's what we have at the moment.

Oh ha ha ha!!! I didn't even notice that, I have never has to flag anyone!